pocock



March 3', 1964 w. E. PococK 3,123,505

PROTECTIVE coATING med oct. 2s, 1961 INVENTOR BMMW ` ORNEYS United States Patent O 3,123,505 PRTECTIVE CATING Walter E. Pocoek, Baltimore, Md., assigner' to Allied Re- Search Products, Enc., Baltimore, Md., a corporation oi Maryiand Fiied Get. 23, 196i, Ser. No. 147,081 S Claims. (Cl. 148-62) This invention relates to the treatment of metals to provide the same with a protective corrosion resistant and abrasion resistant coating which is a substantially clear film deposited from a solution containing less than 2/ g./l. hexavalent chromium calculated as CIOs and an alkali soluble resin, e.g., vinyl acetate resin, glyptal resins, etc. in relatively large amounts. That is, in addition to maintaining the hexavalent chromium (Cr+5) content below 2 g./l. measured as CrOa it is equally important that in the composition, the resin be in amount of l to parts of alkali soluble vinyl acetate resin and 2 to 15 parts of alkali soluble glyptal or alkyd resin (substantially less than 2 g./l. Crt6 measured as CrO3 with l0 to 150 g./l. of the alkali soiuble vinyl acetate resin or to l5() g./l. of the alkali soluble glyptal or alkyd resin. Mixtures of these resins are employed with success, bearing in mind the foregoing teaching as to the relative range of proportions.

Referring to the drawing, a simplified diagram of the process is shown. The flow diagram illustrates the invention used to apply a protective lm to metals such as a zinc plated steel, and to metal alloys such as an aluminum alloy.

Such lilrns also have excellent paint bonding characteristics and more particularly the improved coating composition and method for forming the coating assures absence of heretofore encountered discolouration such as objectionable yellowing.

Previously used organic coatings necessitate the use of organic solvents and where water soluble resins are used, lack the corrosion protective value and wide range of surfaces which can be successfully coated and adhesion characteristics achieved in accordance with this invention.

Moreover, when such prior coatings are baked or force dried, the previously noted disadvantages are aggravated, eg., a bad colour such as a brownish effect is obtained. This is appreciably reduced or eliminated by the present invention. Moreover, where substantially less than 2 g./l. Cris is used and the coating baked or force dried, the previous reduction in abrasion resistance is overcome which is quite important, while at the same time maintaining satisfactory corrosion resistance. The maintenance of both characteristics by the present discovery is of far-reaching signilicance in the industry.

An object of this invention is to provide metals such as steel, aluminum, zinc, cadmium, magnesium, silver, copper, copper alloys, etc. and chemical conversion coatings, eg., chromate, phosphate and oxide coatings on these metals with a clear abrasion resistant and corrosion protective film, which can be used as a iinal finish. Also, it is observed that where substantially below 2 g./l. Cri6 is used, the adhesion of the film to the metal is appreciably enhanced and this naturally is quite desirable. Moreover, there is provided an excellent base for subsequent painting with marked improvement in paint ad- A further object of this invention is to provide a corrosion protective film on metals and alloys which employs appreciably less Crt6 than heretofore considered by prior workers in this iield as necessary, whereby a superior protective film with less Crt6 is obtained, particularly having increased water insolubility which admittedly is most desired.

Where required the lm prepared by this invention,

ice

however, can be removed quickly and easily by immersion in a hot alkaline solution retaining the clean oxidefree surface available as it was prior to treatment. r[his is particularly important where a temporary protective iilm is desired.

in this invention as little as 0.1 g./l. hexavalent chromium with the alkali soluble resin gives the improved and unprophecyable achievement above mentioned in a clear film free of objectionable discolouration such as yellowing and browning and it is found that this eiicect is universally achieved by maintaining the Cr+6 content calculated as Cr03 adequately below 2 g./l.

A particular object of this invention is to obtain these advantages from an aqueous solution containing an alkali soluble polymer or polymers coupled with a chromate corrosion inhibitor or inhibitors operated preferably at room temperature and applied to the metal surface by dip, brush or spray. Higher temperatures up to 260 F. can be used to accelerate drying of the coating. A still further object of this invention is to obtain these advantageous characteristics unexpectedly from a solution containing widely varying amounts of the alkali soluble polymer or polymers and such small amounts of the chromate corrosion inhibitor or inhibitors as heretofore were considered inoperative or ineffective but which I have discovered give vastly improved results in use as well as economy.

it is possible by use of this invention to use organic and inorganic coloring agents to provide color to the lm.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and .scope of the invention will become apparent to those skilled in the art from this detailed description.

With the above objects in view, the invention consists in the improved coating, composition, and method of forming coatings which are hereinafter described and more particularly defined by the accompanying claims.

These objects are unexpectedly accomplished according to the invention by my discovery that contrary to previous views of those skilled in the art, compositions including only a very small amount of hexavalent chromium, CNG, namely less than 2 grams per liter, calculated as CrO3, are successfully useful with a polymeric cornpound from the group of alkali soluble vinyl acetate resins and alkali soluble alkyd resins. The hexavalent chromium inhibitor and the polymeric compound can be applied together to the metal surface or in the event that additional corrosion protection is not necessary but only increased abrasion resistance is desired, an aqueous solution of the polymeric compound can be applied over a corrosion inhibiting chromate conversion coating on the metal.

The compositions of the present invention can be used to apply corrosion resistant coatings to metals such as aluminum, steel, iron, silver, zinc, cadmium, magnesium, copper, copper alloys, etc.

The compositions of the present invention not only are useful to provide protective and corrosion resistant coatings to the metals, but they have improved abrasion resistance over coatings containing corrosion inhibitors, e.g., chromate or the like, Without the polymeric compound.

While sodium hydroxide and potassium hydroxide as well as sodium carbonate and potassium carbonate can be use-d to render the compositions alkaline, if ammonium hydroxide is employed with the alkali soluble vinyl acetate resin or the alkali soluble alkyd resin, the resultsu. E ing coating has improved water resistance in addition to t'ie other advantages previously set forth. Hence, the use of ammonium hydroxide is preferred. in such case, the film is insolubilized simply by drying, pre ably with the aid of heat.

The hexavalent chromium corrosion inhibitor, or inhibitors, used in this invention are water soluble chromium compounds such as sodium, potassium, ammonium chromate, and the correspondir"v dichromates, as well as less soluble complex chromi chromate gels containing both hexavalent and trivalent chromium made, for example, by adding sodium chromate to chromium nitrate under slightly alkaline conditions and very slightly soluble chromium compounds such as zinc chromate and strontium chromate. Such compounds are excellent corrosion inhibitors when used in conjunction with the alkali soluble resins in accomplishing the scope of the invention.

lt should be noted that iilms of the resins by themselves have little or no corrosion protection value, but the combination of resin plus corrosion inhibitors as described gives excellent and unexpected corrosion protection.

l e chromate and polymeric compound can be mixed sa.. Loi

ALKALF. SOLUBLE VlN Y L ACETATE RESIN lVorliing Preferred Range Range Concentration 2 to 10%. 30% ammonia (orequivalent amount 0.05 to 5% (by 0 2 to 1.0% by of other alkali). volume). volume). Solution temperature to 200o F. 70 to 160 F Solution pH. 7 to 11 7.0 to 8.5. Drying temperature 70 to 250 F 14.0 to 100 F.

ALKAL SOLUBLE ALKYD RESN Working Preferred Range Range Concentration 2 to 15 0 5 to 15%. %ammonia(orequivalent amount 0.5 to 1.5% (by 1 to 1.5% (by or" other alkali). volume). volume). Solution temperature 60 to 200 F 7C to 100 F. Solution pH 7 t 7.0 to 8.5

o 11 Drying temperature 70 to 250 F 140 to 160 F.

The chroma-te inhibitor is employed in amounts of hexavalent chromium (Crts), calculated as Cr03, substantially less than 0.2 part and preferably from 0.01 to substantially less than 0.2 part per 100 parts of water. 'Ihis corresponds to amounts of hexavalent chromium (Crt), calculated as CrCE, substantially less than 2 grams and preferably from 0l to substfi.- ally less than 2 grams per liter of solution.

Among the advantages of this invention are that the compositions are relatively low cost and that :excellent results are unexpectedly obtained using very small amounts of the chromate inhibitor. 1f amounts of the inhibitor above the limits of this invention are utilized, no appreciable additional corrosion protection is provided. In fact, greater amounts have undesirable effects on the Coah ing, i.e., water solubility of the film is increased thereby decreasing corrosion resistance, the film tends to discolor toward the yellows and become weakened Ithus making the film unsightly `and less abrasion resistant, and adhesion of the film to metal is decreased thereby shortening the Cil "psoe eifective life thereof. Also, when the films are baked or force dried, all of the above-mentioned undesirable effects are accentuated; for example, the discoloration goes from the yellows toward the browns.

rlihe excellent corrosion resistant coatings of this invention are obtained with ythe small, lalmost minute, amounts of chromates included in the baths taught herein. Above these amounts, is the danger zone wherein the disadvantages discussed above become appreciable and apparent. lt is to be noted, however, that ratios `of chromate to resin content may be Widely varied with goed results so long as the amounts of chromate are within the limits herein prescribed.

As dry compositions there can be used mixtures of polymeric compound and chromate, calculated as CrO3, in the following proportions:

Alkali soluble vinyl aeelto 15 parts tate resin. Alkali soluble alkyrl resin 5 to 15 parts.

ornate 0.01 to substantially 0.01 to substantially less than 0.2 part. less than 0.2 part.

It is `also possible to utilize mixtures of two or more of the polymeric compounds, eg., mixtures of alkali soluble alkyd resins and `an alkali soluble vinyl acetate resin.

ln the instant specification the parts of chromate refer to parts of hexavalent chromium, calculated `as CrO3, except in those specific examples where the parts of the actual chromate compounds, eg., Na2CrO4, are recited.

As previously indicated, the preferred polymeric compounds of the present invention lare alkali soluble vinyl acetate resins. Such resins generally have a major amount of vinyl acetate and la. minor amount of one or more copolymerized monomers in their structure. Thus, the vinyl acetate can be copolymeiized with acrylic acid, maleic anhydride, crotonic acid, etc. The preferred vinyl acetate copolymers `are the Lernac resins (vinyl acetate crotonic acid copolymers) of the Borden Cornpany, includ-ing Lemac 541-10, and Lernac 541-20. Lemac 5411-10l and Lemac 5411-20 differ in their viscosities. Each has been used in the following examples. In general, the alkali soluble vinyl `acetate copolymers have free carboxyl groups or are the salts, eg., sodium potassium and ammonium salts, of such copolymers.

The alkali soluble alkyd resins likewise include free carboxyl groups or the sodium, potassium or ammonium salts threof. A preferred resin of this type is Waterez 1551, manufactured by Reichhold Chemical Company.

Example 1 A test panel of carbon steel electroplated with Zinc was immersed in a solution at F. of the following composition:

Alkali soluble vinyl acetate resin,

Lemar: 541-10 5% (50 g./l.). 30% ammonium hydroxide (by vol. 0.5% (5 Inl/1.). Sodium chromate, Na2CrO4 0.2% (1.2 g./l. CrOg). Water Remainder. pH 7.5.

The test panel, after immersion and without further treatment, was subjected to a temperature of F. for 3 minutes, whereupon the adherent solution dried to a clear water resistant and abrasion resistant film. When the panel so treated was exposed for 168 hours in a standard salt spray test cabinet using a 20% salt solution, only slight zinc corrosion products appeared at the edges of the panel. After 500 hours exposure in the salt spray, about 1A of the surface ofthe panel showed Zinc corrosion products, but no rusting. A simil-ar test panel, zinc plated but not otherwise treated, showed heavy zinc corrosion products over the entire surface after only 24 hours exposure in the salt spray, and severe yrusting after 500 hours exposure.

A similar test panel was immersed in the coatingorming solution and allowed to dry at room temperature. A clear coating was formed as before, having approximately the same corrosion resistant quality, but slightly less water resistance and abrasion resistance than when dried at 160 F.

A similar test panel was immersed in the coatingforming solution and dried at 250 F. A clear coating was formed having somewhat greater -water resistance and 'abrasion resistance, ibut slightly less corrosion resistance than Iwhen dried at 160 F.

Example 2 Example 1 was repeated but using 10% alkali soluble vinyl acetate resin. Results `obtained were substantially the same.

Example 3 A test panel of carbon steel was cleaned of surface soil by immers-ion in an alkaline soak cleaner, rinsed and then yimmersed in the solution of Example 1. After immersion and without `further treatment, the panel was subjected to a temperature of `160" F. for 3 minutes, whereupon a clear water resistant and abrasion resistant film was :formed as in Example l. The panel showed only slight rusting after `8 hours salt spray exposure, and moderate rusting after 20 hours exposure. A similar steel panel which was cleaned but not otherwise treated, was severely rusted over the entire surface after only' 2 hours salt spray exposure.

When a steel panel, treated as :described above, and a steel panel that had been cleaned only, were painted with a standard baked enamel, a marked improvement was observed in the adherence of the enamel to the treated as compared to the untreated panel.

Example 4 Similar results to those in AExample 3 were obtained by replacing sodium chromate with ammonium chromate.

Example 5 lSimilar results to those in Example 3 were obtained by increasing the amount of sodium chromate to 0.25%,

corresponding to 1.5 grams of CrOa per liter of solution. Example 6 A number of zinc-plated steel support frames were subjected, in a production operation, to the following procedure.

(l) immerse 20 seconds at 80 F. in:

Chromic acid, CrO3 70 g./l. Sodium sulfate, NagSO.,= 10 g./l. Nitric acid, 40 B. 30 Inl/l. Water Remainder.

(2) Rinse in water.

(3) timmerse 5 seconds at 80 F. in:

Caustic soda, NaOH 22.5 g./l. Water Remainder.

(4) Rinse in water. (5) immerse in:

Lemac 541- resin 5% (50 g./l.). S o d iu m chromate,

Na2CrO4 0.05% (0.3 g./l. CrO3). 30% ammonium hydroxide 0.5% (by vol.; 5 ml./l). Water Remainder.

(6) Dry 5 minutes C@ 140 F.

Steps l-4 are Well known in the art as a bright chromate treatment, Iwhich forms a bluish-bright corrosion resistant chromate film on the Zinc surface.

Steps 5 and 6 resulted in the formation of a clear abrasion resistant and water resistant coating, which by virtue of :its ability to mask a slight-coloration normally obtained from steps 1-4, gave an improved appearance. The coating obtained from steps 5 and 6 also caused a marked improvement in resistance to nger staining and atmospheric corrosion as compared to the finish obtained from steps 1-4 only.

Example 7 Example 9 A steel panel was cleaned and then coated with a standard zinct phosphate conversion coating (Bonderite D-180, Parker Rust-Proof Co.) followed by conventional chromic acid rinse, and dried. A second steel panel was similarly phosphate-coated, except that the panel was immersed in the following solution, instead of the chromic acid, then dried at F.:

Alkali soluble Vinyl acetate resin, Lemac 541-20 5% (50 g./1.).

30% ammonium hydroxide 0.5% (by vol., 5 ml./l.). Sodium chromate,

Na2CrO4 0.2% (1.2 g./l. CrOg).

The two panels were subjected to the standard 20% salt-spray test. After 20 hours exposure, the panel treated in the above solution showed only slight rusting while the panel that had been chromic acid rinsed only was severely rusted.

Example 10 A piece of steel chain having a conventional black oxide coating was immersed in the alkali soluble vinyl acetate resin solution of Example 9 and dried at 160 F. After 2 hours salt spray exposure, the chain was only slightly rusted, while a similar piece of chain with the black oxide coating only was severely rusted after 2 hours salt spray.

Example 11 A steel panel was cleaned, rinsed and immersed in the following solution:

Alkali soluble alkyd resin (Waterez 1551) 5%(50 g./l.). 30% ammonium hydroxide 0.3% (by vol.; 3 ml./l).

Na2CrO4 0.05% (0.3 g./l. CrOg).

The panel was then subjected to a temperature of 250 F. for 5 minutes whereupon a clear abrasion resistant and water resistant coating was formed. After 2 hours salt spray exposure, the panel was only slightly rusted. A similar steel panel which had been cleaned only was severely rusted after only l hour salt spray exposure.

Example 12 Example 9 was repeated, except using 0.15% sodium chromate, corresponding to 0.9 gram of CrO3 per liter of solution. Results obtained were substantially the same.

Example 13 `A steel test panel was cleaned, immersed in the following solution and dried at 250 F.:

Alkali soluble alkyd resin (Waterez 1551) 5% (50g./l.). 30% ammonium hydroxide 0.2% (by vol.; 2 nil/1.). Sodium chromate, Na2CrO4 0.2% (1.2 g./l.).

A clear abrasion resistant coating was formed. After twenty-four hours salt spray exposure, the panel was only slightly rusted, while a similar panel that had been cleaned only was severely rusted in only two hours exposure.

Example Z4 Example 13 was repeated except using 0.28% sodium chromate, corresponding to 1.7 grams of Cr03 per liter of solution. Corrosion resistance as shown by salt spray was further improved.

Example 15 A Zinc-plated steel test panel was immersed in the solution or Example 13 and dried at 250 F. A clear abrasion resistant coating was formed. After twenty-four hours salt spray exposure, the panel was only slightly corroded, while a similar panel without the coating was severely corroded after only two hours exposure.

Example 16 Example 15 was repeated except using 0.29% sodium chromate, corresponding to 1.8 grams of Cr03 per liter of solution. Corrosion resistance as shown by salt spray was further improved.

Example 17 A copper test panel was cleaned, immersed in the solution of Example 13, and dried at 250 F. A clear abrasion resistant film was formed. After 7 hours salt spray exposure, the panel was only slightly corroded while a similar panel, that had been cleaned only, was severely corroded after 4 hours exposure.

Example 19 A test panel of FS-l alloy magnesium was alkaline cleaned, pickled to remove oxide, rinsed in water, immersed in the solution of Example l, and dried at 160Q F. A clear abrasion resistant coating was formed. The corrosion resistance of the panel was markedly greater than that of a similar panel that had been cleaned and pickled only, as indicated by 24 hours salt spray exposure.

Example Zinc metal sheet was dipped into an aqueous solution containing:

Chromic acid (Cr03) grams/liter 7 Sodium sulfate do 1 Nitric acid, 42 Be ml/liter 3 The sheet was allowed to remain in the solution until a visible conversion coating was formed (about 15 seconds). The sheet was then removed, rinsed in water and immersed in an aqueous solution containing:

Alkali soluble vinyl acetate resin, Lemac 541-20 5% (50 g./l.). 30% ammonium hydroxide 0.5% (by vol.; 5ml/1.). Sodium chromate 0.02% (0.1 g./l. C103).

The sheet was dried at F. whereupon a clear coating was formed having corrosion and abrasion resistance superior to that of the chromate conversion coating itself.

This application is a continuation-in-part of application Serial No. 711,776, tiled January 29, 1958, now abancloned.

What is claimed is:

1. An aqueous composition for coating metals containing from 0.1 to substantially less than 2 grams per liter of hexavalent chromium calculated as Cr03 and a polymeric compound selected from the group consisting of 10 to 150 grams of alkali soluble vinyl acetate resin and 20 to 150 grams of alkali soluble alkyd resin per liter of aqueous composition and sullicient alkali to dissolve the resin.

2. A composition according to claim 1 wherein the polymeric compound is an alkali soluble vinyl acetate resin.

3. A composition according to claim 1 wherein the polymeric compound is an alkali soluble alkyd resin.

4. A composition according to claim 1 wherein the alkali is ammonium hydroxide.

5. A composition according to claim 1 wherein the hexavalent chromium is present in a compound selected from the group consisting of sodium chromate, potassium chromate, ammonium chromate and the corresponding dichromates and complex chromium chromate gels.

6. A composition according to claim l wherein the hexavalent chromium is present in sodium chromate.

7. A method of imparting a corrosion-resistant coating a metal which comprises subjecting the metal to an aqueous solutionl containing a polymeric compound selected from the group consisting of alkali soluble vinyl acetate resins and alkali soluble alkyd resin, suflicient alkali to dissolve the polymeric compound, and from 0.1 to substantially less than 2 grams per liter of solution of hexavalent chromium calculated as CrO3.

8. A method according to claim 7 wherein the solution contains 1.2 grams per liter of hexavalent chromium calculated as CrOa.

References Cited in the le of this patent UNITED STATES PATENTS 2,162,618 Izard June 13, 1939 2,184,310 Meigs Dec. 26, 1939 2,921,858 Hall Ian. 19, 1960 3,053,692 Pocock Sept. 1l, 1962 3,053,785 Rosenbloom Sept. 11, 1962 

7. A METHOD OF IMPARTING A CORROSION-RESISTANT COATING A METAL WHICH COMPRISES SUBJECTING THE METAL TO AN AQUEOUS SOLUTION CONTAINING A POLYMERIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI SOLUBLE VINYL ACETATE RESINS AND ALKALI SOLUBLE ALKYD RESIN, SUFFICIENT ALKALI TO DISSOLVE THE POLYMERIC COMPOUND, AND FROM 0.1 TO SUBSTANTIALLY LESS THAN 2 GRAMS PER LITER OF SOLUTION OF HEXAVALENT CHROMIUM CALCULATED AS CRO3. 